Kyte et al. (2003) and Lowe et al. (2003) are to be congratulated on their study of Early to Middle Archean impact fallout units in the Barberton greenstone belt, South Africa, representing a major advance in the understanding of the nature of early Precambrian Earth. Here I point to some of the implications and outstanding questions arising from these studies.
Kyte et al. (2003) estimate asteroid diameters from mass balance calculations of Ir abundances and 53Cr/52Cr isotopes (Shukolyukov et al., 2000), consistent with Melosh and Vickery's (1991) calculations based on condensation spherule (microkrystite) maximum sizes, which suggest asteroids several tens of kilometers large. Applying projectile/crater scaling factors of 1/10 to 1/15, impact craters on a scale of several hundred kilometers in diameter are implied. The largely mafic (chlorite, Cr-sericite and stilpnomelane-dominated) composition of impact spherules and the absence of shocked quartz in the Archean impact fallout units suggest mafic/ultramafic crustal (simatic) loci of these craters, consistent with studies of impact fallout units in the Pilbara craton (Simonson et al., 1998; Vickers, 2003, personal commun.). The evidence for Archean analogues of lunar maria (Green, 1972; Glikson, 2001), on a similar scale as Crisium or Serenitatis, is inconsistent with uniformitarian models of the early crustal evolution.
Inferences from microkrystite spherule chemistry to projectile composition need to take into account vapor fractionation processes. Kyte et al. (2003) suggest the Barberton greenstone belt spherules display chondritic platinum group element distribution patterns, modified through hydrous alteration and consequent loss of the relatively mobile Pd and Au. However, similar depletion of low-condensation point platinum group elements (e.g., Pd at 3237 °C and Au at 3130 °C) as compared to the high-condensation point platinum group elements (e.g., Ir at 4701 °C and Pt at 4100 °C), reflected by (Pd/Ir)N ratios <1.0 (N is chondrite normalized), is also observed in 2.56 Ga (Simonson et al., 1998) and 2.47–2.50 Ga (Vickers, 2003, personal commun.) Hamersley Basin spherules, Western Australia. These patterns are contrasted with the relative enrichment of Pd relative to Ir in associated mafic and ultramafic volcanic rocks, which show (Pd/Ir)N ratios >1.0. The only other example for low Pd/Ir ratios of which I am aware is harzburgite depleted in low-melting point components. Since both the spherule beds and juxtaposed mafic/ultramafic greenstones were subjected to similar greenschist facies conditions, the depletion in volatile platinum group elements shown by the spherules may conceivably be related to original fractionation in the impact-released cloud from which the spherules condensed, with consequent loss in low-condensation temperature (volatile) components. Such fractionation is consistent with observed depletion in Cr (condensation temperature 2945 °C) relative to V (condensation temperature 3682 °C) in 2.47–2.50 Ga spherule-rich beds in the Hamersley Basin (Vickers, 2003, personal commun.).
The locally extreme enrichment in platinum group element levels in Barberton greenstone belt spherules, particularly the S3 and S4 units observed by W. Taylor and A.Y. Glikson, 2000, personal commun.). Extensive alteration of the chromites by chlorite is observed, with consequent heterogeneous secondary distribution patterns of the platinum group elements.
The depositional environment of the Barberton greenstone belt spherules interpreted in terms of a transition from shallow-water fandelta environment in the southern Barberton greenstone belt to deep-water environments in the northern Barberton greenstone belt (Lowe et al., 2003; Kyte et al., 2003) requires consideration. As winnowing and reworking of spherule-bearing beds by shallow-water currents can be expected to result in destruction of the delicate, originally glassy spherule structures, unless protected by rapid burial and subsidence, preservation of spherules in shallow-water environments is less likely. In the Pilbara craton spherule-bearing units are invariably hosted by below–wave base siltstone, carbonate, chert, and banded ironstone, which show little or no shallow-water current reworking. Breccia and conglomerate-hosted spherules (in the ?2.56 Ga Carawine Dolomite and 3.47 Ga Apex Basalt) are interpreted in terms of large-amplitude tsunami disturbance of the deep (below normal wave base) sea bed (Vickers, 2003, personal commun.). Conspicuous features of these units are downward-injected sedimentary veins containing spherules, as below S3 in the Barberton greenstone belt (Lowe et al., 2003) and in the Carawine Dolomite spherule-bearing megabreccia. The almost invariably superior preservation of spherules within these veins has been attributed to extreme hydraulic pressures and consequent dilation affected by the tsunami (Vickers, 2003, personal commun.). Rapid subaqueous subsidence of host greenstone sequences, indicated by thick pillow lava sequences, suggests a high rate of subsidence of the depositional basin. It may be necessary to test whether current disturbances in the southern Barberton greenstone belt formed in a shallow fandelta environment, or represent a deeper seafloor environment disturbed by deep-amplitude southwest- to northeast-directed tsunami waves.